Over the last two decades coiled tubing has proven increasingly useful in oil and gas field applications, including workovers, drilling, logging and well stimulation. These versatile coiled tubing units typically comprise a continuous length of several thousand feet (possibly up to 20,000 feet) of steel tubing, capable of withstanding pressures in the order of 10,000 psi, capable of being repeatedly coiled and uncoiled from a truckable spool, and capable of being injected and withdrawn from oil and gas wells without killing the well.
Continuous coiled tubing can be run and operated using coiled tubing injectors in a pressurized well. Coiled tubing drilling can be carried out without drilling mud. Fluids can be passed through the tubing for a variety of purposes. Being flexible, coiled tubing has particularly lent itself to horizontal well applications where segmented pipe has difficulty maneuvering and bending, such as through a rapid build out section. As a result in horizontal wells coiled tubing has grown to serve a variety of purposes, from drill pipe to production tubing.
The term "coiled tubing" has a generally understood meaning in the field. "Coiled tubing" lengths are understood to comprise several hundred or several thousand feet of continuous, uniform outer diameter tubing, coilable on a "truckable spool" and injectable in a bore by means of a coiled tubing injector. The continuous lengths are typically, although not necessarily, manufactured of steel having a longitudinally welded seam. Resin and fiber polymer materials are being proposed for continuous tubing. More typically today successive lengths of steel tubing are welded to create a desired unit length. "Coiled tubing" is expected to be able to withstand significant pressure differentials, such as at least 1,000 psi and preferably closer to 10,000 psi, and to be sufficiently corrosive resistant and heat resistant to withstand exposure to common hydrocarbons "downhole."
The term "truckable spool" implies a spool or reel having an outside diameter of no more than six meters, and preferably less, so that the spool can be transported by truck over land and highways to a site. Sufficient flexibility to be reeled on a "truckable spool" implies an ability of the tubing to be repeatedly coiled and uncoiled from such a spool or reel. The ability to be injected in a bore implies that the tubing, while flexible, is also sufficiently stiff that lengths of several hundred or thousand feet can be continuously thrust, or injected, in bores using coiled tubing injection equipment.
The present invention comprises, in one aspect, a new variety of coiled tubing, concentric coiled tubing and/or insulated coiled tubing, sometimes referred to herein as a coiled tubing "composite." The composite fits the definition of "coiled tubing" in that the composite is sufficiently flexible for reeling on a truckable spool and sufficiently stiff to be injected in a bore. The composite satisfies length and pressure and corrosion resistance expectations as well.
Establishing that concentric coiled tubing and/or insulated coiled tubing can be built opens up new and improved uses for coiled tubing. One such use for the novel insulated coiled tubing comprises integration of the tubing into apparatus for the thermal production of hydrocarbons.
Cost, or added cost, is a key factor in the economics of the thermal recovery of hydrocarbons and minerals from viscous underground formations. The versatility and reusability of novel insulated coiled tubing, together with its flexibility and compact size, may so favorably affect profit margins as to render thermal recovery from many additional formations economical. Novel insulated coiled tubing further permits consideration of simultaneous heating and production from a single well.
Heavy oil and tar sands comprise significant existing resources for liquid hydrocarbons to the extent that they can be economically produced. As a general matter, heavy oil from tar sand and bitumen deposits must be heated to reduce the oil or mineral viscosity before it will flow, or to enhance flow. The predominant method for heating utilized in the field is the injection of a hot fluid, usually steam, from the surface, although electrical heating is also practiced. In the absence of a feasible and cost effective method for simultaneously injecting steam and producing fluid from a single well, one common industry practice has been for steam to be injected through a designated "injection" well and heated hydrocarbons produced simultaneously from surrounding "producing" wells. An alternate single well commercial practice has been for steam to be injected intermittently into a production well. In this so-called "huff and puff" system the single well alternates in function from an injection well to a producing well. Certain single well thermal recovery systems have been proposed where the same well is used for simultaneous heating and production. Two such systems are described in U.S. Pat. No. 5,285,846 to Mohn and U.S. Pat. No. 4,019,575 to Pisio, although the Pisio system also uses a second adjacent well for producing a steam drive. Both Pisio and Mohn teach the use of concentric, traditionally segmented pipe joints with their system. Neither teaches the use of coiled tubing. Nor do they teach the importance of insulated tubing designed to minimize the loss of heat to the well bore prior to reaching the formation to be heated. In fact, Pisio appears to teach away from such art.
Insulated segmented pipe string, or jointed thermal pipe, has been developed for use in some thermal applications. It is recognized that insulated jointed tubing limits the thermal energy lost to the overburden during the transport of a heated fluid underground, thereby delivering more thermal energy to an oil or mineral bearing formation. However, insulated segmented tubing, or jointed thermal pipe, is expensive, can be difficult to maneuver in horizontal wells, is slow to run and pull and may unduly restrict annular wellbore space by virtue of the upset of the tubing's joints.
One object of the present invention is to provide a cost effective, versatile maneuverable alternative to jointed thermal pipe. Such apparatus can facilitate simultaneously injecting high quality steam and thermally recovering hydrocarbons from existing single wells. Other downhole and above wellhead uses for the novel concentric and/or insulated tubing are anticipated now that its construction has been proven. The tubing can be specifically tailored to meet the needs of a single application, or it can be designed to meet general off-the-shelf standard specifications. It is to be understood that the particular materials to be selected and the design to be utilized can vary, according to the application limitations to be satisfied.
The apparatus, comprising coiled tubing, should be particularly adaptable to horizontal wells. It may permit variations in thermal recovery methodologies to be developed, such as a practice in which injection sites are varied. Electrical insulation as well as thermal insulation can be offered by insulated coiled tubing, permitting strategies utilizing electrical transmission in lieu of or as well as fluid transmission.
In establishing the viability of insulated coiled tubing the present inventors participated in the implementation of a successful steam injection project. Production in a heavy oil producing well was dramatically increased utilizing the first prototype insulated concentric coiled tubing (ICCT) string. The first prototype ICCT provided a net thermal conductivity roughly equivalent to that of conventional insulated production tubing at a lower overall cost to the producer.
The test project commenced with a search for a cost effective way of delivering high quality steam to a horizontal wellbore. The objective was to implement a cost effective steam delivery system that would permit continuously heating a heavy oil reservoir. This would warm the high viscosity oil allowing it to readily flow into a slotted liner. The oil could then be pumped to surface through a parallel production string within the same wellbore. This new heavy oil production system has been named the SW-SAGD system and is described in more detail in pending patent application U.S. Ser. No. 08/420,038 entitled SINGLE HORIZONTAL WELLBORE GRAVITY DRAINAGE ASSISTED STEAMFLOOD PROCESS AND APPARATUS, hereinafter referred to as "SW-SAGD patent application." Conventional insulated production tubing, or jointed thermal pipe, had been considered for the job, but was determined to be uneconomical and impractical for use at the particular site.
Attempting to satisfy the needs of the test project led the parties eventually to design, construct and install a 1745 meter ICCT string. A loose fill insulation material was chosen for the CCT annulus due to its low thermal conductivity and apparent suitability as a material that would enable the assembly of continuous lengths of concentric coiled tubing. To ensure a uniform distribution of insulating material around the inner coiled tubing, and to centralize an inner coiled tubing within an outer coiled tubing, special ceramic/steel centralizers were developed for the first prototype. These centralizers retained the mechanical and thermal strengths of steel while adding the low thermal conductivity and high service temperatures of ceramic materials. The centralizers, located at strategic locations along the tubing, were designed to resist the mechanical and thermal stress anticipated during initial assembly of the concentric coiled tubing string, as well as during thermal expansion induced under operating conditions and during future coiled tubing operations or workovers.
The first insulated concentric coiled tubing string, consisting of a 650 meter vertical section and a 1050 meter horizontal section including build section, was installed for testing in a heavy oil lease near Provost, Alberta, in January, 1995. Steam injection commenced at a break-in temperature of about 100.degree. C. to 150.degree. C. Although the maximum steam injection design temperature was 350.degree. C., initial temperatures were reduced to preclude thermal shock effects. Over the following days steam injection temperatures were increased to about 300.degree. C. Production levels began to climb steadily and target production rates were exceeded. After the establishment and propagation of a steam chamber within the oil bearing formation, in accordance with the SW-SAGD method taught in the SW-SAGD patent application, the well continued to produce at levels well above those expected and at steam volumes considerably less than design values.
Further novel strings are being constructed and tested in other thermal Enhanced Oil Recovery (EOR) production pilot projects, including additional continuous injection and production wells. The technology offers excellent potential for significant reductions in heating costs and increased oil production. Utilization of insulated and/or concentric coiled tubing with conventional "huff and puff" steam injection processes may offer advantages for operators. Traditional steam assisted gravity drainage and other techniques may also find use for the novel apparatus.
The present invention comprises insulated coiled tubing and/or concentric coiled tubing. Two sets of factors come into play in designing particular units of such tubing. The first set of factors comprises the degree of insulation required or the rigor of the insulation limitations imposed, thermal or electrical or both. The second set of factors comprises the cost effectiveness of the product for the intended use. The most cost effective embodiment of the invention for the initial project, and hence the preferred embodiment disclosed in depth below, comprised dual coiled tubing, concentrically arranged, having insulation in the annulus, referred to as ICCT. Insulated coiled tubing, however could be formed based upon a single coil with insulation attached, either without or within. Forms of insulation exist which could be used for wrapping or filling or lining the interior of such coils. The present inventors reviewed and evaluated and simulated many such alternate embodiments. Given the required thermal insulation characteristics, cost effectiveness limitations and necessity to minimize time and effort set by the initial project, the particular embodiment of the ICCT or insulated concentric coiled tubing disclosed below was preferred for the initial prototype. It is to be understood that other end uses might render an insulated single coil, either insulated within or without, a viable, cost effective embodiment, as well as concentric coils utilizing many alternative insulating strategies. For instance, fluids could be transmitted alternatively, or simultaneously, through a concentric coil annulus.